An insulated gate bipolar transistor (IGBT) is one example of a power semiconductor device. In the IGBT, for example, a p-type collector region, an n-type drift region, and a p-type base region are provided on a collector electrode. And, a gate electrode is provided in a trench that penetrates through the p-type base region and reaches the n-type drift region, with agate insulating film interposed between the gate electrode and the trench. In addition, an n-type emitter region connected to an emitter electrode is provided in a region that is adjacent to the trench in the surface of the p-type base region.
In the IGBT, a positive voltage higher than a threshold voltage is applied to the gate electrode to form a channel in the p-type base region. Then, holes are injected from the collector region to the n-type drift region at the same time as electrons are injected from the n-type emitter region to the n-type drift region. Then, a current having the electrons and the holes as carriers flows between the collector electrode and the emitter electrode.
It is effective to increase the carrier concentration of the n-type drift region in a state in which the IGBT is turned on in order to reduce the on-resistance of the IGBT. In contrast, in a case when the IGBT is turned off, if the discharge of carriers in the n-type drift region is delayed, the turn-off time increases and switching loss increases. Double gate driving has been proposed as a method for reducing both on-resistance and switching loss. The double gate driving is a technique in which two gate driving systems are provided and the driving timing of two gates is changed to reduce the switching time and the switching loss of an IGBT. Therefore, it is possible to reduce both on-resistance and switching loss.